Ethylene oxide and other olefin oxides are important industrial chemicals used as a feedstock for making such chemicals as ethylene glycol, propylene glycol, ethylene glycol ethers, ethylene carbonate, ethanol amines and detergents. One method for manufacturing an olefin oxide is by olefin epoxidation, that is the catalyzed partial oxidation of the olefin with oxygen yielding the olefin oxide.
In olefin epoxidation, a feed containing the olefin and oxygen is passed over a bed of catalyst contained within a reaction zone that is maintained at certain reaction conditions. The epoxidation catalyst generally contains the catalytically active species, typically a Group 11 metal (in particular silver) and promoter components, on a shaped carrier material.
During the epoxidation, the catalyst is subject to a performance decline, which represents itself by a loss in activity of the catalyst and selectivity in the formation of the desired olefin oxide. In response to the loss of activity, the epoxidation reaction temperature may be increased such that the production rate of the olefin oxide is maintained. The operation of commercial reactors is normally limited with respect to the reaction temperature. When the applicable temperature limit has been reached, either the production rate of the olefin oxide is reduced or the production of the olefin oxide has to be interrupted for an exchange of the existing charge of epoxidation catalyst for a fresh charge.
U.S. Pat. No. 4,529,714 (the “'714 patent”) describes a process for regenerating silver containing carrier catalysts used in the preparation of ethylene oxide. The process comprises treating a deactivated catalyst with a solution comprising a potassium, rubidium, or cesium compound and a reducing agent. In the Example of the '714 patent, an ethylene oxide catalyst was regenerated after approximately four years of service, during which the “catalytic activity” (i.e., selectivity) diminished from an initial 81.5 percent (at 218° C.) to 76.7 percent (at 247° C.). '714 patent, col. 4, 11. 16-18; see also Table spanning col. 3-4. The maximum increase in “S %” reflected in that Table at any temperature is only 3.2% (an increase from 76.7 to 79.9).
To a large extent, the selectivity of an epoxidation catalyst determines whether an epoxidation process is economically attractive. For example, a one percent improvement in the selectivity of an epoxidation process can produce a substantial reduction in the yearly operating cost of a large scale ethylene oxide plant.
A need exists for processes for producing rejuvenated epoxidation catalyst with greater selectivity and/or greater activity.